This invention relates to lamps and particularly to arc discharge lamps. Still more particularly, the invention relates to arc discharge lamps employing a ceramic arc tube, a shield or shroud and mounting means for mounting the arc tube within the shroud.
Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. Recent advances in the art have employed a ceramic arc tube construed, for example, from polycrystalline alumina. It is with the latter type that this invention is particularly concerned. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material that may include mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud that comprises a generally tubular, light-transmissive member, such as quartz, that is able to withstand high operating temperature. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends.
In those lamps using an arc tube made from quartz or fused silica or like material, the arc tube has a generally tubular body sealed at the ends by a pinch seal. The pinch seals provide a flattened area on the arc tube that lends itself to receiving a mounting structure that both positions the arc tube within the shroud or shield and allows the entire structure to be mounted upon a suitable frame within an envelope.
The shroud or shield has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
In lamps using ceramic arc tubes, mounting the arc tube within a shroud has proven difficult and expensive. The ceramic arc tube has a tubular, often bulbous body with ceramic, cylindrical capillaries extending therefrom. The capillaries are relatively small, often having diameters of 3 mm or so, and contain the electrodes.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the mounting of arc tubes within shrouds.
It is yet another object of the invention to achieve these objects in an inexpensive manner.
These objects are accomplished, in one aspect of the invention, by a spring clip that comprises a base in a first plane and having an aperture centrally located therein. U-shaped projections are provided on the base and are formed from first and second upstanding walls and third and fourth upstanding walls, one at each end of the base. A first lip extends orthogonally between the first wall and second wall in a second plane and a second lip extends orthogonally between the third wall and fourth wall, also in the second plane. The second plane is substantially parallel to the first plane but spaced therefrom. Extensions are formed with the first wall and the third wall and project away from them. The extension are in substantially the same plane as base and at least one flag can be formed with at least one of the extensions. The flags project away from the extensions in a plane transverse to the first and second planes and allow the clips to be attached to a frame as will be seen below.
The clips are used to provide an assembly of an arc tube mounted within a shroud. The assembly then comprises a light source having a center and projecting, opposite ends arrayed along a longitudinal axis, the ends being cylindrical in cross-section. A tubular shroud surrounds the light source and is coaxial with the longitudinal axis. The shroud has two ends, and a given wall thickness. A pair of spring clips is provided, one at each end of the shroud, by fitting the U-shaped projections of the clips over the wall of the shroud. An arc tube is positioned between the clips with the ends of the arc tube encompassed within the apertures in the base of the clips. The clips are economical to manufacture, easy to use and avoid ergonomic problems that were associated with prior methods. They also lend themselves to automated assembly, further reducing the cost of the lamps with which they are used.